LHS Cooling system fans

Hiya Folks,

Here's a puzzler for you. The culprit: 1997 LHS 3.5 Liter. When idling,
the temperature climbs higher than normal. Checked fans and they don't
start until higher temp reached then run until engine cools some but not to
normal level. (They cut out early.)

Problem comes and goes (intermittent). Checked all connections (computer to
relays; computer to temp sensor; etc.). Replaced both high and low speed
relays. Bypassing relays on relay board, the fans start at once.

Thought it might be temp sensor but then read in Chrysler's official service
manual that the temp gauge in car gets signal from same sensor. I reason
that if sensor is off, the guage wouldn't read high either.

Any ideas?

Puzzled

P.S. Problem ocurred with two different computers installed. (Replaced for
other reason.)

Hank NB wrote:

> Hiya Folks,
>
> Here's a puzzler for you. The culprit: 1997 LHS 3.5 Liter. When idling,
> the temperature climbs higher than normal. Checked fans and they don't
> start until higher temp reached then run until engine cools some but not to
> normal level. (They cut out early.)
>
> Problem comes and goes (intermittent). Checked all connections (computer to
> relays; computer to temp sensor; etc.). Replaced both high and low speed
> relays. Bypassing relays on relay board, the fans start at once.
>
> Thought it might be temp sensor but then read in Chrysler's official service
> manual that the temp gauge in car gets signal from same sensor. I reason
> that if sensor is off, the guage wouldn't read high either.
>

Are you sure? SOME Chrysler vehicles always used two separate sensors-
one for the temp guage, and one for the PCM. I own a 3.5L LH car, but
have never had a coolant sensor problem and so never bothered to check
in any detail.

That said, do the fans truly run on low speed when you bypass the low
speed relay, and high speed when you bypass the high speed relay? The
reason I ask is because there are failure modes in one of the fan motors
(the larger of the two) that will cause no fan operation on low speed,
and either normal operation on high speed or only one fan running on
high speed (depending on which brush fails in the motor). You should be
able to see both fans spinning and also hear the difference in pitch
between low and high speed. The behavior is much like what you get when
the large fan motor fails and you get no fans at all until the PCM calls
for high speed fan, then you get reduced fan power.

"Steve" > wrote in message
...
> Hank NB wrote:
>
>> Hiya Folks,
>>
>> Here's a puzzler for you. The culprit: 1997 LHS 3.5 Liter. When
>> idling, the temperature climbs higher than normal. Checked fans and they
>> don't start until higher temp reached then run until engine cools some
>> but not to normal level. (They cut out early.)
>>
>> Problem comes and goes (intermittent). Checked all connections (computer
>> to relays; computer to temp sensor; etc.). Replaced both high and low
>> speed relays. Bypassing relays on relay board, the fans start at once.
>>
>> Thought it might be temp sensor but then read in Chrysler's official
>> service manual that the temp gauge in car gets signal from same sensor.
>> I reason that if sensor is off, the guage wouldn't read high either.
>>
>
> Are you sure? SOME Chrysler vehicles always used two separate sensors- one
> for the temp guage, and one for the PCM. I own a 3.5L LH car, but have
> never had a coolant sensor problem and so never bothered to check in any
> detail.
>
> That said, do the fans truly run on low speed when you bypass the low
> speed relay, and high speed when you bypass the high speed relay? The
> reason I ask is because there are failure modes in one of the fan motors
> (the larger of the two) that will cause no fan operation on low speed, and
> either normal operation on high speed or only one fan running on high
> speed (depending on which brush fails in the motor). You should be able to
> see both fans spinning and also hear the difference in pitch between low
> and high speed. The behavior is much like what you get when the large fan
> motor fails and you get no fans at all until the PCM calls for high speed
> fan, then you get reduced fan power.
>
>


Thank you Steve,

I did check the fan operation at both speeds, by-passing the relays and both
fans perform as they should. According to the FSM the temp sensor signal
going to the computer (PCM) directs both the fan control and the temp guage.
Also I could tell that when the fans did kick in, it was through the
low-speed relay because I had my finger on it and could feel it click.

Have done a little more research and found an old post (Aug 2003), via
Google newsgroups, on this same group that suggests there are three more
sensors to consider. They a Intake air temperature sensor; Output speed
sensor (not sure what that means); and Transmission oil teperature sensor.
I'll have to dig back into my FSM (Factory Service Manual) to see if I can
find anything on those and also where they are located.

Looking forward to any further input from you folks out there. )

-Hank